Several games for the Nintendo DS use the system's onboard microphone for commands and communication. You can talk to your dog in Nintendogs, shout legalese in Phoenix Wright, and even voice chat with friends over the Internet with Metroid Prime: Hunters and Pokemons Diamond and Pearl. Unfortunately, doing all these things usually means awkwardly crouching over the portable's tiny microphone--located on the front edge of the original Nintendo DS and in the hinge area between the two screens on the DS Lite.
Or you can pick up the Nintendo DS Headset. The accessory retails for under $15, and is currently available only in white. It's an extremely simple one-ear headset with a boom mic for voice chat or voice commands in compatible games. The attached cord is about 40 inches long, and because it's terminated with the proprietary DS remote plug, you won't be able to use it as a headphone for anything else. It slips over the main section of your ear, in the same fashion as some cell phone headsets.
After plugging it in, the headset supplants the DS speaker and microphone. The microphone carries voice decently, and can certainly help in the few DS games that are voice-enabled. Fellow Pokemon Diamond players could hear us just fine, and not having bring the DS up to our mouths was a relief. There is one caveat: in games like Spectrobes, where you're asked to blow into the mic, the headset won't be a good choice. We didn't have any on hand to test, but the boom mic doesn't even reach the mouth, so it's safe to say it won't work very well, if it all.
The one-ear earphone is serviceable. While the Nintendo DS doesn't have a very complicated audio processor--a majority of its music and sound effects sound little better than Super Nintendo-era beeps and boops--the one-ear design means you'll lose the stereo effects you'd hear with a standard two-ear headphone design (or even the DS stereo speakers).
Of course, any standard set of headphones with a 1/8-inch (3.5mm) plug will work with the DS--though they'll lack the boom mic on the Nintendo Headset. But there's at least one other full-service alternative: the $20 Turtle Beach Ear Force D2 headphones include a microphone and a standard two-ear stereo configuration--and they're even available in multiple colors.
In the final analysis, the Nintendo DS Headset works as advertised--though it has a very narrow mission. It's only very handy if you like to voice chat in Metroid Prime: Hunters or Pokemon (or any other future game that supports online multiplayer). If you play a lot of voice-command-based games, you can probably make due with the DS' onboard microphone; whether you have a headset or not, you're still going to be shouting orders into your DS and generally looking like a psychotic. But hey--that's part of what makes the DS so fun.